1. Field of the Invention
This invention relates to cooling equipment for electronic systems, e.g., fans, and more particularly, to measuring the rotational speed of a fan.
2. Description of the Related Art
Fans are often used to evacuate warm air from enclosures in which electronic systems are contained. For example, most computer systems include one or more cooling fans to aid in circulating the air inside the enclosures and for maintaining the temperature inside the enclosures within an acceptable range. The increased airflow provided by fans typically aids in eliminating waste heat that may otherwise build up and adversely affect system operation. Employing cooling fans is especially helpful in ensuring proper operation for certain central processing units (CPUs) with relatively high operating temperatures.
Control of fans in a system typically involves a fan control unit executing a fan control algorithm. A fan control algorithm may determine the method for controlling one or more fans that are configured to evacuate warm air from a system enclosure. For example, the fan control algorithm may specify that a fan's speed should be increased or decreased dependent upon a detected temperature. Such control algorithms may also involve turning off a fan if the temperature is deemed cool enough to do so.
Fans often include a tachometer output that provides a signal indicative of the current speed of the fan. The tachometer signal may be used to determine whether the fan is operating properly. Often, fans used for CPU and/or computer system cooling have a three-wire interface with wires for power, ground, and the tachometer signal. Fan drive systems often use a signal generator that provides a Pulse Width Modulated (PWM) signal to drive an external circuit that controls the voltage between the power and ground interfaces of the fan, which in turn controls the speed of the fan. Signal generators that provide PWM signals are useful because they provide a digital control for the pulse width of a signal. The fan is typically powered only for the duration of the pulse. Between pulses power to the fan is turned off, although the fan is typically still spinning during this time. The duty cycle of the PWM pulse train currently being provided to the fan determines the fan's speed.
One problem associated with using signal generators that provide PWM signals to drive fan circuits is that the tachometer circuitry associated with the fan does not receive power during the time the fan is turned off between the PWM pulses. As a result, the tachometer signal output by the fan may not accurately represent the current fan speed during the time between pulses. Similarly, once the fan is turned off, the tachometer signal does not indicate the speed of the fan as the fan spins down. One technique that is currently used to measure fan speed in these situations involves using an analog filtering system to measure the back Electromotive Force (EMF) inserted into the fan tachometer signal by the rotating fan. Another problem associated with using signal generators that provide PWM signals to drive fan circuits is that multiple cooling zones create the need for multiple sensors and multiple fans, thereby causing the need for complex management schemes in fan operation management.
One digital technique used for measuring fan speed includes stretching the PWM signal pulse to insure that a valid tachometer signal remains asserted until the speed of the fan has been determined in case the duty cycle of the PWM signal would not permit such a measurement. There are, however, disadvantages associated with such techniques, including low accuracy, especially at low PWM duty cycles, fan speed surges resulting directly from stretching the PWM pulse and unwanted fan noise resulting from the fan speed surges. Another drawback is that the stretching typically dominates at low PWM duty cycles. In other words, the fan's speed, which is measured in revolutions-per-minute (RPM), is effectively controlled by the stretching of the PWM pulse and not by the duty cycle itself of the PWM signal. Such techniques generally lead to complicated implementations with significant impact on die size, and are typically prone to electrical noise sensitivity.
Many other problems and disadvantages of the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.